1. Field of the Invention
The present invention relates to the field of photoelectric devices. It more particularly concerns a transparent conductive layer deposited on a substrate intended for a photoelectric device in thin layers. A layer of this type is most often called, by specialized, a TCO (transparent conductive oxide) layer or electrode. The invention also concerns a method for producing this electrode.
2. Description of the Related Art
Application of the present invention is particularly interesting for producing photovoltaic cells intended for the production of electrical energy, but it is also applicable, more generally, to any structure in which light radiation is converted into an electrical signal, such as photodetectors.
In the current state of the art, photoelectric devices in thin layers, typically having a thickness smaller than 10 μm, are made up of a substrate that is transparent or not, flexible or stiff, and, deposited on this substrate, a photoelectrically active layer made up of an inorganic semi-conductor material or, more rarely, an organic one, and contacted on both sides by two electrodes, at least one of which is transparent. The semi-conductor layer is generally formed by the stacking of a p-type layer, an intrinsic-type active layer, and an n-type layer, together forming a p-i-n or n-i-p junction. The material used is primarily amorphous silicon or hydrogenated microcrystalline. In the case of an organic photoelectrically active layer, this is generally formed by stacking a p-type layer and an n-type layer. The material used is then, for example, a polymer.
In order to limit the production costs of the photoelectric device, the intrinsic active layer must be relatively thin (between 100 nm and several microns). However, such a layer leads to a weak quantity of light absorbed, particularly for indirect gap materials, such as microcrystalline silicon and, as a result, a reduced effectiveness. To offset this effect, it is therefore necessary to increase the optical path of the light as much as possible within the intrinsic layer. This is generally done through the use of a textured TCO substrate or layer, making it possible to diffuse or diffract the incident light, and thereby to increase the length of the path of the light in the active layer.
Document DE 197 13 215 describes a solar cell whereof the substrate is covered with a TCO layer, advantageously in zinc oxide (ZnO), formed by cathode sputtering into an argon atmosphere from a ZnO target doped with aluminum. In order to grant a roughness to this TCO layer, normally without asperities, it is etched either through a chemical method using an acid solution, or through an electrochemical method (anodic etching or reactive ion etching). The etching can be done during or after deposition of the TCO layer.
This type of method does, however, suffer several drawbacks. First, the cathode sputtering equipment and the necessary targets are relatively expensive, which substantially overloads the price of the cells thus produced. Secondly, etching of the TCO layer is delicate. It must therefore be dosed carefully, failing which one obtains, for the TCO layer, a surface morphology, in particular large craters, which is not favorable to optical trapping, as well as interruptions which are not very favorable to good later growth of the photoelectric layer.
Document JP 62-297 462 proposes depositing a TCO layer by evaporation—and not chemically—and interrupting this operation to soften the surface of the layer by argon plasma etching.
Such an approach, applied to the production of a photovoltaic cell, would lead, due to the deposition by evaporation, to a film of very low roughness, clearly insufficient, in any case, to grant it a an acceptable optical trapping capacity for this application. The action of an argon plasma on the deposited layer would again serve to reduce the roughness of the layer, which would make it practically incapable of providing the optical trapping function of a photovoltaic cell.
One indication of the optical trapping capacity of a layer is provided by the “Haze factor”, which assumes the value of 0% when no part of the incident light is diffused and the value of 100% when all of the light received is diffused. Of course, the “Haze factor” of a solar cell must have as high a value as possible, typically 10% minimum.
The values provided in the abovementioned JP document, however, are 2 to 5% before the action of the argon plasma and 0.5% after treatment, respectively. These values clearly show that argon plasma etching of a layer deposited by evaporation is not aimed at the field of photovoltaic cells.
Document EP 1 443 527 describes a textured TCO layer having a surface morphology formed of a sequence of flat hollows which possess a number of micro-asperities having a base from 0.1 to 0.3 μm, a height from 0.05 to 0.2 μm and a pitch (distance between the peaks) from 0.1 to 0.3 μm. Such micro-asperities do not, however, lend themselves particularly well to good later growth of the photoelectric layer. Moreover, due to their small size, they do not increase light trapping very much in the range of interest (red and infra-red). Moreover, the fact that the hollows are flat has the drawback of increasing reflection of the light and, because of this, decreasing the light injected into the photovoltaic device, thereby reducing the photo-generated current accordingly.